Conventionally, a tag identifying system is known (Japanese Unexamined Patent Publication No. 8-316888) that it has a half duplex communication channel provided between an interrogator and two or more tags which exist within the range of the interrogator and an identification of the tag is not told to the interrogator at first, but the tags are activated when the interrogator polls a signal having a single frequency, and each tag can un-activate itself.
In this tag identifying system, it is constituted so that the first broadcast from interrogator broadcasts the first signal which are used for activating all the tags within the range of the interrogator and the first inquiry message, which contains the first value showing number of the tags within the range of the interrogator.
Since a memory means established for every tag responds the first reference message, an identifying display which is not identified by the interrogator is memorized in a memory.
Moreover, it is constituted so that the first remains tag formation means equipped to every tag responds to a reception of the first reference message performs calculation, which is based on the first value, a random bit for every tag and a memorized identifying display, and when the calculation produces a predetermined corresponding result, by un-activated tags, the first set of activated tags which is formed.
It is constituted so that the tags is un-activated after the first un-activated means which identified the first set of each activated tags changed the identifying display, memorized in the memory of the first set of each tag, showing that an identification is successful with the interrogator.
Moreover, it is constituted so that the second broadcast means carries out the broadcast of the second reference message, which contains the second signal which activates all the tags within a range of an interrogator and the second value which shows an un-identified tag within a range of the interrogator, and the second remaining tag formation means and the second un-activated means form the second set of activated tags by using the second value instead of the first value, the number of identified tags are increased.
Furthermore, it is constituted so that the second broadcast means, the second remaining tag formation means, and the second un-activated means is performed the processing using the inquiry message which has the following signal and the following value, until identification of all the tags within a range of an interrogator finishes.
In the above-described tag identifying system, tags are divided into a small group, and by turning OFF power supply of the tag which does not belong to the group, one group of the tags is identified at once so that a radio power can be saved.
Each tag puts itself into a group by performing calculation from a parameter memorized in itself and a parameter of an interrogator.
However, in the tag identifying system shown in Japanese Unexamined Patent Publication 8-316888 of the former, if two or more tags are overlapped, a mutual inductance may arise between an antenna coil of each tag, and a resonance frequency of the tag may change.
For this reason, whether an interrogator forms an alternative magnetic field in a circumference or oscillates a radio (a radio wave which an un-overlapped tag resonates) the tag resonates from a transceiver antenna of the interrogator, the tag stops oscillating and stops supplying a radio power to a RFID device of the tag.
Consequently, the interrogator had the fault which becomes unable to identify the overlapped tags.
The purpose of the present invention is to offer an identifying system for overlapping tags which can identify the tags certainly whether two or more tags are overlapped or a tag and a metal plate are overlapped.